1. Field of the Invention
The present invention relates to a catalyst for hydrogenation of living polymer. More particularly, to an organic titanium catalyst of homogeneous phase having high activity in a reaction in which unsaturated double bonds in conjugated diene units of living polymers are selectively hydrogenated. Also, the present invention is concerned with a method for hydrogenating living polymer, using the catalyst.
2. Description of the Prior Art
Copolymers of conjugated diene monomers, such as 1,3-butadiene and isoprene, with copolymerizable aromatic vinyl monomers, such as styrene, are widely used as elastomers. The block copolymers of conjugated diene monomer and aromatic vinyl monomer are thermoplastic elastomers and can be used as modifiers for polyolefin and polystyrene resins to prepare transparent resins having impact resistance. These copolymers can be vulcanized by the presence of olefinic double bonds in polymers, but have disadvantages that durability and resistance to oxidation against oxygen and ozone in atmosphere are lowered. Thus, such copolymers can be applied only within a limited range where they are not exposed to the air or the outside.
Generally, it can be achieved to improve durability and resistance to oxidation of such copolymers by hydrogenating the olefinic double bonds in the polymers partially or wholly saturating the double bonds.
Many methods for hydrogenation of polymers having olefinic double bonds have been reported. In general, they can be divided into two type methods: one using a heterogeneous catalyst of which metals, such as nickel, paladium, platinum, ruthenium, etc. are dispersed in supports, such as carbon, silica, aluminum, calcium carbonate, etc.; and the other using a homogeneous catalyst of Ziegler catalyst consisting of organic salts of nickel, cobalt, iron, chromium, titanium, or the like and reducing compound such as an organoaluminum or the like, or organometal compound, such as rhodium and titanium.
In the case of using the heterogeneous catalyst, hydrogenation is comprised of dissolving olefinic polymers in appropriate solvents and contacting with hydrogen in the presence of the heterogeneous catalyst. In this method, the contact between reactant and catalyst cannot easily be carried out owing to the steric hindrance and relatively high viscosity of polymer. Also, owing to the strong adsorption between polymer and catalyst, it is very difficult for other unsaturated polymers to access to the active point. For complete addition of hydrogen to the remaining unsaturated polymers, the hydrogenation reaction is required to be carried out at high temperature and at high pressure in the presence of a large amount of catalyst. Such conditions often result in the decomposition of the polymer and gelation thereof and, in the case of the copolymer of conjugated diene and aromatic vinyl monomer, saturation of the aromatic double bonds is simultaneously proceeded. It is therefore difficult to perform selective hydrogenation of olefinic polymer. In addition, it is very difficult for the catalyst to be physically separated from the hydrogenated polymer solution and some heterogeneous catalysts are virtually impossible to be completely removed due to their strong adsorption for the polymers.
In contrast with the heterogeneous catalyst, the homogeneous catalyst shows high activity in hydrogenation and thus, high hydrogenation yield is expected even with a small amount of catalyst under reaction conditions at a low temperature and a low pressure. In addition, hydrogenation can be selectively executed at the double bonds, exclusive of the aromatic moiety, in the copolymer of conjugated diene and aromatic vinyl hydrocarbon under appropriate hydrogenation conditions.
Hydrogenation or selective hydrogenation of conjugated diene polymers is described in many literatures.
U.S. Pat. Nos. 3,494,942, 3,634,594, 3,670,054 and U.S. Pat. No. 3,700,633 disclose use of catalysts containing metal of Periodic Table VIII group or precursors thereof for hydrogenation or selective hydrogenation of ethylenically unsaturated pwymers and ethylenically unsaturated polymers with aromatic groups. In said patents, metal of Periodic Table VIII group, especially, nickel or cobalt compound is formulated with a reducing agent, such as aluminum alkyl, to prepare useful catalyst. Also, there is described in the prior arts that aluminum alkyl is preferred reducing agent, but I-A, II-A and III-B group metals of Periodic Table, especially, lithium and magnesium alkyls or hydrides thereof are effective reducing agents. The mole ratio of I-A, II-A or III-B group metal to VIII group metal is in the range of 0.1:1 to 20:1 and preferably 1:1 to 10:1.
U.S. Pat. No. 4,501,857 suggests that the double bonds present in conjugated diene polymers can be selectively hydrogenated by hydrogenating the polymers in the presence of at least one of bis(cyclopentadienyl)titanium compounds and at least one of lithium hydrocarbon compounds.
U.S. Pat. No. 4,980,421 describes that alkoxy lithium compounds formulated with bis(cyclopentadienyl)titanium compounds and optionally at least one of reducing organometallic compound such as aluminium, zinc and magnesium compounds have analogous hydrogenation activity, wherein alkoxy lithium compounds could be directly added or be added as a reaction mixture of organic lithium compounds with alcoholic or phenolic compounds.
U.S. Pat. No. 4,673,714 showed that bis(cyclopentadienyl)titanium compounds could preferably hydrogenate the double bonds of conjugated diene without using alkyl lithium. It was mentioned that this titanium compound was bis(cyclopentadienyl)titanium diaryl compounds and that an advantage of this catalyst system is no use of lithium hydrocarbon compound.
U.S. Pat. No. 5,039,755 discloses a hydrogenation method in which conjugated diene monomers are polymerized or copolymerized with a polymerization initiator of organic alkali metal, to produce living polymers. The polymerization of the produced living polymer is terminated by adding hydrogen. The selective hydrogenation of the double bonds in conjugated diene units of the terminated polymer is carried out with (C.sub.5 H.sub.5).sub.2 TiR.sub.2 (R=arylalkyl).
However, since the homogeneous catalyst appears that its hydrogenation activity is largely changed depending on the reduction state of the catalyst, it is difficult to obtain hydrogenated polymer having high yield and reproducibility. In addition, the active ingredients of the catalyst are tend to be changed into inactive ones by impurities in the reaction system. So, the impurities serve as a factor which lowers the reproducibility of the catalyst. Such inactivation of the homogeneous catalyst causes a serious problem in the hydrogenation of polymer for improving durability and resistance to oxidation.
In general, hydrogenation using homogeneous catalysts is not sufficiently rapid in hydrogenation rate and its activity depends on the reduction state of catalyst and impurities in the reaction system. Hence, the conventional homogeneous catalysts are problematic in applying the hydrogenation reaction of polymer on an industrial scale. Therefore, there has been strongly required a catalyst which is able to show high hydrogenation rate and produce highly hydrogenated polymer, without being affected by impurities of the reaction system and the preparation condition of catalyst.
The above-mentioned problems, however, cannot be solved by the catalysts suggested in the literatures and patents published thus far.